AnOrthorhombicPhaseofSuperhardo-BC4N

A potential superhard o-BC4 N with Imm2 space group is identified by ab initio evolutionary methodology using CALYPSO code. The structural, electronic and mechanical properties of o-BC4N are investigated. The elastic calculations indicate that o-BC4N is mechanically stable. The phonon dispersions imply that this phase is dynamically stable under ambient conditions. The structure of o-BC4N is more energetically favorable than o-BC4N above the pressure of 25.1 GPa. Here o-BC4N is a semiconductor with an indirect band gap of about 3.95 eV, and the structure is highly incompressible with a bulk modulus of 396.3 GPa and shear modulus of 456.0 GPa. The mechanical failure mode of o-BC4N is dominated by the shear type. The calculated peak stress of 58.5 GPa in the（100）[001] shear direction sets an upper bound for its ideal strength. The Vickers hardness of o-BC4N reaches 78.7 GPa, which is greater than that of t-BC4N and bc-BC4N proposed recently, confirming that o-BC4N is a potential superhard material.